The issue with this is payments themselves can’t be tagged to a project - only invoices, estimates, purchases and purchase orders can. But any payments made to them are taken into account for the cash based reporting of the projects.
What may help, is we expanded the project tagging almost a year ago, so clients and suppliers automatically include specific project tags. What this allows you to do is set a specific project tag against a client or supplier, and any new invoices created for them automatically includes them. More information on that, here:
I hope that helps?